Nasa has revealed an ambitious plan to capture an asteroid and tow it into the
moon's orbit so it can be studied and even be visited by astronauts.

Under the plan, an unmanned spacecraft will travel to a 9 metre-wide asteroid and secure it in a harness before flying it into a steady orbit, probably near the moon.

There, astronauts would be able to study the pieces of the rock in an effort to "understand the origins of the solar system and inform decisions about how to conduct missions to distant planetary bodies".

The mission could take place as soon as 2020, Nasa has said.

Studying an asteroid in this way would allow scientists to learn more about the universe, how it was created while also providing information to help defend Earth from future asteroid strikes.

The plan first emerged last week but was detailed in a budget statement from Nasa on Wednesday. It is expected to cost about $78 million.

He said the plan would not pose any danger to Earth and that it would test "the technologies and capabilities required if in the future there is a need to move a hazardous asteroid."

The Nasa budget for 2014 is $17.7bn, down $50m from 2012.

But Nasa hopes that the asteroid mission could provide a vital stepping stone towards future missions to other planets such as Mars.

William Gerstenmaier, associate administrator for human exploration and operations, said: “The mission to find, capture and redirect an asteroid robotically, and then visit it with astronauts to study it and return samples takes advantage of expertise across all of NASA in an integrated approach to exploration.

"This mission will give us valuable experience we need in deep space operations to send humans to more distant destinations in the solar system, including Mars."